Degree

Doctor of Philosophy (PhD)

Department

Petroleum Engineering

Document Type

Dissertation

Abstract

In large scale subsurface injection of carbon dioxide (CO2) as obtainable in carbon sequestration programs and in environmentally friendly hydraulic fracturing processes (using supercritical CO2), long term rock-fluid interaction can affect reservoir and seal rocks properties which are essential in monitoring the progress of these operations. The mineralogical components of sedimentary rocks are geochemically active particularly under enormous earth stresses, which generate high pressure and temperature conditions in the subsurface. While geomechanical properties such as rock stiffness, Poisson’s ratio and fracture geometry largely govern fluid flow characteristics in deep fractured formations, the effect of mineralization can lead to flow impedance in the presence of favorable geochemical and thermodynamic conditions. Simulation results suggested that influx-induced mineral dissolution/precipitation reactions within clay-based sedimentary rocks can continuously close micro-fracture networks, though injection pressure and effective-stress transformation first rapidly expand the fractures. This experimental modelling research investigated the impact of in-situ geochemical precipitation at 50°C and 1,000 psi on conductivity of fractures under geomechanical stress conditions. Geochemical analysis were performed on different samples of shale rocks, effluent fluid and recovered precipitates both before and after CO2-brine flooding of crushed shale rocks at high temperature and pressure conditions. Bulk rock geomechanical hardness was determined using micro-indentation. Differential pressure drop data across fractured composite core were also measured with respect to time over a five a day period. This was used in estimating the conductivity of the fractured core. Three experimental runs per sample type were carried out in order to check the validity of observed changes. The results showed that most significant diagenetic changes in shale rocks after flooding with CO2-brine reflect in the effluent fluid with calcium based minerals dissolving and precipitating under experimental conditions. Major and trace elements in the effluent fluid (using ICP-OES analysis) indicated that multiple geochemical reactions are occurring with almost all of the constituent minerals participating. The geochemical composition of precipitates recovered after the experiments showed diagenetic carbonates and opal (quartz) as the main constituents. The bulk rock showed little changes in composition except for sharper peaks on XRD analysis, suggesting that a significant portion of amorphous content of the rocks have been removed via dissolution by the slightly acid CO2-brine fluid that was injected. However total carbon (TOC) analysis showed a slight increase in carbon content of the bulk rock. Micro-indention results suggested a slight reduction in the hardness of the shale rocks and this reduction appear dependent on quartz content. The differential pressure drop, its 1st derivative and estimated fracture conductivity suggests that reactive transport of dissolved minerals can possibly occlude fracture flow path at varying degree depending on equivalent aperture width, thereby improving caprock integrity with respect to leakage risks under CO2 sequestration conditions. An exponential-natural logarithm fit of the fracture conductivity can be obtained and applied in discrete fracture network modelling. The fit yielded lower and upper boundary limits for fracture conductivity closure. Higher temperature and pressure conditions of experimental investigations may be needed to determine the upper limit of shale rock seal integrity tolerance, under conditions that are similar to sequestration of CO2 into deep and hot sedimentary rocks.

Date

8-28-2017

Committee Chair

Harvey, Craig M

DOI

10.31390/gradschool_dissertations.4105

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